1. Field of the Invention
The present invention relates generally to an information processing method, information processor, computer program, and storage medium for storing the program, and more specifically to an information processing method for hiding information to protect digital data, and an information processor, computer program and storage medium for same.
2. Description of the Related Art
Under the recently promoted standard for digitization of text, image and voice data, prevention of illegal copying of digital data and association of digital data with metadata are required.
For document images on printed materials, digital data and printed material are combined. And so, in order to protect such combined digital data, the access control system must be capable of linking both the digital data and printed material. Under these circumstances, information-hiding techniques such as digital-watermark information embedding have been employed.
In digital-watermark information embedding, part of original data is modified so that it is imperceptible to a human being, additional information is then embedded in the image, voice, or text data.
Specifically, when digital-watermark embedding is applied to multi-valued images, redundancy of pixel density is employed. However, since binary images such as document images have little or no redundancy, digital-watermark embedding is difficult to apply. In view of this problem, other methods that use characteristics peculiar to document images have been proposed.
These methods include moving the baseline of lines, modifying the length of spaces between words, modifying inclination of characters by rotating them, and modifying the length of spaces between characters. By such minimal modification, additional information can be embedded in a document image.
As a specific example, a method for modifying the length of space between characters is described with reference to FIGS. 9 and 10.
FIG. 9 shows part of a document image before digital-watermark information is embedded therein. FIG. 10 shows part of the document image after digital-watermark information has been embedded in part of the document image shown in FIG. 9. Space lengths P0, S0, P1, and S1 between characters in FIG. 9 are changed to space length P0′, S0′, P1′, and S1′ shown in FIG. 10 by embedding digital-watermark information therein. In FIGS. 9 and 10, five characters and four spaces between them are shown. In this case, two spaces are allocated to 1 bit, and thus four spaces can be embedded with two bits.
For example, P>S represents “1” and P<S represents “0”. In this state, if the character between P0 and S0 is shifted to the left and the character between P1 and S1 is shifted to the left in FIG. 9, P0′<S0′ and P1′<S1′ are satisfied in FIG. 10, which shows part of the document image after embedding. Accordingly, a bit string (2 bits) of “1” and “0” can be embedded.
The above-described method for modifying the length of space between characters is disclosed in, for example, “Electronic document data hiding technique using inter-character space”, The 1998 IEEE Asia-Pacific Conf. On Circuits and Systems, 1998, pp. 419-422, by King Mongkut University.
In such digital-watermark information embedding, information cannot be embedded in some portions of the document due to the positions of circumscribed rectangles. For example, as shown in FIG. 11, when the length of each of P and S is 1 pixel, the circumscribed rectangle can neither be moved to the right nor the left because the rectangle would contact the adjoining rectangle.
In such a case, the watermark extracting side cannot determine whether this portion of the document contains embedded digital-watermark information or whether it contains the original unmodified portion in which information cannot be embedded.
That is, in the known art, it is difficult for both the embedding and extracting sides to recognize the same character string, which includes digital-watermark information.